The link between space flight and osteoporosis

link between space flight and osteoporosis
Bedridden patients and astronauts who are exposed to zero gravity for prolonged time share something in common: progressive bone loss.

Osteoporosis results from gradual loss of bone density, so that the skeleton becomes weaker and more susceptible to fractures. Like patients with osteoporosis, astronauts who spend longer periods of time in space also experience bone loss, but at a much faster rate.

When astronauts first travelled in space doctors eagerly awaited their return to see how zero gravity had affected them. The most obvious impact was that their muscles had wasted away, and it was realized soon after that so too had their bones.1,2

According to NASA, astronauts who spend many months on a space mission can lose, on average, 1 to 2 per cent of bone mass each month. They typically experience bone loss in the lower halves of their bodies, particularly in the vertebrae (spine) and the leg bones. The proximal femoral bone (thigh bone) loses 1.5 percent of its mass per month, or roughly 10 percent over a six-month stay in space, with the recovery after returning to Earth taking at least three or four years. The loss of bone mass also triggers a rise in calcium levels in the blood, which increases the risk of kidney stones.

What researchers have learned by studying the impact of space travel on bones is very relevant for bedridden patients. Patients who remain immobile in bed over longer periods of time experience rapid and progressive bone loss. They lose bone density because they don't exercise muscles that would otherwise build skeletal strength through motion. Studies with ‘terranauts’ (healthy, young Earth-bound volunteers who lie flat without exercising for extended periods of time) have shown that completely immobilized bones can lose up to 15% of mineral density within three months. Excessive training is then required to restore not only muscle mass, but also bone density and bone strength - however the latter cannot be restored completely.2

To help overcome the effects of bone loss while in orbit, astronauts have to engage in physical exercise for two and a half hours a day, six times a week during their stay in space. Although this does not completely eliminate the risk of bone loss, it does help to reduce it.3

The take home message for ordinary Earth-bound people is that exercise and bone maintenance are inextricably linked. Anyone who is at risk of osteoporosis, or who has been bedridden for some time, must make a special effort to build up muscle and bone strength.

One key way to do that is by engaging in weight-bearing and resistance exercise on a regular basis. Jogging, stair-climbing, Nordic walking, using muscle-strengthening equipment at the gym, yoga, dancing, playing tennis, etc.: so many different kinds of physical activity (if safe and appropriate for an individual's level of physical fitness) can help build and maintain that all-important muscle and bone mass.

LYB logoThis article appeared in our bimonthly Love Your Bones newsletter sent to IOF members.

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